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VACCINE. This term has undergone a change in meaning from that which it received at the hands of Jenner, the discoverer of vaccine inoculation for the preventive treatment of smallpox. This was a little over a hundred years ago and before the existence of modern bacteriology. Yet the principle which Jen ner enunciated and which he hut dimly and imperfectly understood is the same which in more recent years has been extensively elabo rated by Pasteur, Koch and the entire great school of bacteriologists. It is the principle that by introducing into the animal body cer tain substances of known character, immunity or protection will be conferred for a longer or shorter period against the attack of disease producing germs which may be within that body or which may subsequently gain access to it. A vaccine or vaccine virus, according to Jenner, was the contents of a pustule upon the skin, especially upon the udder of a cow or calf suffering with cowpox, whia when intro duced into the lymph or blood circulation of a human being sensitive to smallpox, rotected him in some way or other from that disease or arrested the disease if it was already in its incipient stage. A vaccine as understood by bacteriologists is a preparation composed of a great number of living attenuated bacteria or a smaller number of very virulent ones, or their products, including split bacteria, bacterial toxins and filtrates of fluid cultures. The re production of the bacterial cells is prevented by the use of a sufficient degree of heat, and the vaccine is preserved by the addition of a suitable antiseptic. Vaccines are conveniently prepared in the form of liquids which may be clear or turbid and with more or less abundant precipitate. Vaccines are used to prevent or cure diseases of many kinds by introducing them into the body through the mouth, the sub cutaneous tissue, within the skin, within the substance of the muscles or directly into the blood circulation through a vein. The theory of vaccination presumes the presence within the blood of what are known as antibodies. Metch nikoff refers to them as phagocytes; they are also known as antigens. The antibodies are antagonistic to disease germs which have ob tained access to the circulating blood. Their function is to attract some disease germs and repel others. When there is mutual attraction the germ is enveloped and absorbed by the antibody and the tendency to the disease of which the germ is the specific cause is thus limited and arrested. The object of the vac cine which is introduced into the circulation is to stimulate the healthy tissues to produce new antibodies and so increase the chance of de stroying the invading disease germs.

It was not until 1874 that it was discovered by Traube that normal blood had bactericidal properties, this being followed by the important discovery of Gehring that the serum of an artificially immunized animal may transmit im munity to other animals. This is called its bacteriolytic power and follows the injection into the animal of suitable bacteria. An in jection of a suitable dose of vaccine into an animal is followed by a diminution of the bac teriolytic power of the blood, this being known as the negative phase of resistance. The larger the dose the more significant the clinical fea tures of this stage, including rise of temperature and other physical disturbance. After this comes the positive phase when the antibacterial power of the blood and the resistance to bac terial influence are increased. By repeating these injections in increasing dosage the anti bacterial and antitoxic power of the blood of the animals thus treated is increased for the disease germs against which the treatment has been directed. It is by this method of experi ment; particularly in horses, that the antitoxin of diphtheria has been made available. One great merit in the use of vaccines as a thera peutic agent is that they are comparatively harmless, except in those who are in a weak and toxic condition from tuberculosis or some other infectious disease.

Vaccines are of various kinds. Autogenous vaccines are derived from cultures of bacteria obtained from the blood or discharges or secre tions of one who is sick or injured and are injected into the same individual upon the theory that the stimulation to the development of antibodies will be greater than if the vaccine is obtained from an external source; they are of particular value in obstinate or unusual in fections in which an exact diagnosis has not been made. Their production requires con siderable time and is not available for very acute diseases. Stock vaccines are those which are made from representative types of particu lar organisms and are preserved for use accord ing as those types are indicated as means of treatment; they are less costly to make than autogenous vaccines but are also distinctly less effective. The polyvalent form of stock vac cines contains different strains of the same bac terial species or one strain from different sources and is the most efficient variety of stock vaccine. Mixed vaccines are those which contain two or more strains from different spe cies and are intended for mixed infections or those in which there is a variety of infecting bacteria. Consult Kolmer, J. A., 'A Practical Text-Book of Infection, Immunity, and Specific Therapy' (Philadelphia 1915).